Non-volatile memory devices that retain stored data in the absence of power are pervasively used in many electronic products including cell phones, tablets, personal computers, personal digital assistants, and the like. Unfortunately, many non-volatile memory devices have limitations that make them unsuitable for use as primary storage for these products including higher cost and lower performance when compared to volatile memory devices such as dynamic random access memory (DRAM). Examples of non-volatile memory devices include read-only memory (ROM), flash memory, ferroelectric random access memory (FRAM), resistive random access memory (RRAM), phase change memory, and the like.
RRAM memory devices are a focal point for current development. In some instances, RRAM memory devices are implemented as a three-dimensional (3D) crosspoint memory array having vertically stacked and interconnected layers. Each layer may comprise an array of RRAM memory elements, formed at intersections of bit lines and word lines placed orthogonal to each other. RRAM memory devices formed on a layer may be interconnected to RRAM memory devices formed on other layers.
RRAM memory devices may further be implemented as horizontal 3D crosspoint memory arrays and vertical 3D crosspoint memory arrays. Horizontal 3D crosspoint memory arrays having 1D-1R (one diode, one resistive element) often use unipolar diodes as select devices. These unipolar diodes, however, cannot service popular bipolar RRAM memory devices. Vertical 3D crosspoint memories use a shared transistor to select between multiple RRAM devices. Sneak paths in the array limit the block size, however, which adversely impacts manufacturing costs. A need exists, therefore, for an improved RRAM memory device.